Encapsulation of phase change material (PCM) into nanoparticles has received increasing attention due to its potential applications in heat transfer fluids and thermal energy storage. However, the conventional batch-based synthesis processes such as nanoprecipitation, sol-gel, and ultrasonic techniques have shortcomings such as limited batch-to-batch reproductivity and nonhomogeneous chemical and physical properties. Micro- and millifluidic technologies have been studied over the past decades as a potential platform for nanoparticle synthesis. In this work, a co-flow millifluidic device where vortex and turbulence can be formed was developed to achieve a rapid and precise control of the mixing of two fluids with a potential application in nanoparticle encapsulation. The device was fabricated by soft lithography method in which the master model was made by 3D printing technology. Parylene conformal coating was utilized to deposit a thin parylene C layer on the polydimethylsiloxane (PDMS) channel walls to prevent the absorption of organic solvents and molecules into PDMS material. The flow behavior phase diagram was developed by operating the millifluidic device under different flow rates of the inner and outer fluids, including laminar, transition, and turbulence flow regimes. At last, polystyrene (PS) nanoparticles (NPs) were synthesized using the developed device to demonstrate its capability in nanoparticle synthesis. The comparison of the number and volume distributions of the PS nanoparticles with those from bulk synthesis method shows that the co-flow millifluidic device can synthesize NPs at a much smaller size and more uniform size distribution.

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